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INTRODUCTION
Nitrogen (N) is the most important nutrient and
occupies the highest requirement position in plant
nutrient compared to the other essential nutrients (1).
N is widely distributed throughout the lithosphere,
atmosphere, hydrosphere and biosphere. Only a very
small part of this element is presents in the soil, as
Phosphate solubilization characteristics of efficient nitrogen fixing soil Azotobacter strains
Rahim Nosrati1, 2, Parviz Owlia1*, Horieh Saderi1, Iraj Rasooli1, 2, Mohammad Ali Malboobi3
1Molecular Microbiology Research Center (MMRC), Shahed University, Tehran, I.R. Iran. 2Department of Biology, Faculty of Science, Shahed University, Tehran, I.R. Iran. 3National Institute of
Genetic Engineering and Biotechnology (NIGEB), Tehran, I.R. Iran
Received: December 2013, Accepted: May 2014
ABSTRACT
Background and Objectives: Azotobacter is a diazotroph bacterium reported to possess various plant growth-promoting
characteristics. The aim of this study was to isolate Azotobacter strains capable of fixing nitrogen and effectively hydrolyzing both organic and inorganic P
i compounds.
Materials and Methods: In this study, soil samples collected from a diverse range of slightly alkaline soil types were
screened for Azotobacter isolates. The inorganic and organic phosphate solubilization potentials of twenty competent
phosphate solubilizing Azotobacter isolates were assessed. Variations were noted in the solubilization potentials.
Result: Three isolates, identified as Azotobacter vinelandii strains O2, O4 and O6, were able to fix atmospheric N2
effectively. The nitrogenase activity of these isolates ranged between 158.6 and 326.4 C2H
4h-1vial-1 (ethylene). Bacterial
growth rates and phosphate solubilization activities were measured quantitatively under various environmental conditions.
A close association was evident between phosphate solubilizing ability and growth rate as an indicator of active metabolism.
All three phosphate solubilizing bacteria (PSB) were able to withstand temperature as high as 45°C, high concentration of
NaCl (upto 5%) and a wide range of initial pH from 5 to 10 while hydrolyzing phosphate compounds actively.
Conclusion: Azotobacter vinelandii strains O2, O4 and O6 are superior candidates for biofertilizers that may result in the
reduction of chemical nitrogen and phosphate fertilizers leading to increase crop production.
Keywords: Azotobacter, Biofertilizer, Nitrogen fixation, Pisolubilizing
*Corresponding author: Parviz Owlia Ph.D.
Address: Molecular Microbiology Research Center
(MMRC), Shahed University, No. 31, Abdollahzadeh St.,
Keshavarz Blvd, P.O. Box 14155-7435, Tehran, I.R. Iran.Tel: +98-21-88964792E-mail: [email protected]
Volume 6 Number 4 (August 2014) 285-295
both inorganic and organic forms. The inorganic N
in the soil is only a small fraction of the total soil
N (2). The total N contents of surface mineral soils
normally ranges between 0.05 and 0.2 percent, in
most cases less than five percent, is directly available to plants, mainly as nitrate (NO
3
2-) and ammonium
(NH4
+). Organic N slowly becomes available through
mineralization as well (1). Most of the N is to form of
dinitrogen (N2) in the earth’s atmosphere. N
2 cannot
be used directly by plants and animals; therefore, N2
must be first ‘fixed’ or converted into a form such as ammonia before being consumed (2-4). N enters the
biosphere through biological nitrogen fixation (BNF) by microorganisms (3,5). N fixation is a process of reducing atmospheric nitrogen to ammonia,
catalyzed by the nitrogenase enzyme complex (6-8).
286 IRAN. J. MICROBIOL. Vol. 6, No. (August 2014), 285-295 http://ijm.tums.ac.ir
In the fields, phosphorus (P) is the second most essential macro nutrients for plant growth and
development that is absorbed only in soluble forms of
phosphate ion (Pi), HPO
4
2- or H2PO4-. In fact, total soil
phosphorus content is well beyond plant needs (400–
1,200 mg/kg); however, it is mostly immobilized in
the forms of organic and inorganic compounds such
that the only a small fraction of P (1 mg kg−1 or less)
is available for plant growth (for reviews see (3, 9)).
Even, a large portion of added chemical Pi fertilizer
is immobilized rapidly in soil to its insoluble forms
(10, 11). Therefore, the availability of Pi is highly
dependent on chemical compositions and biological
processes occurring in the soil, particularly within
the rhizospher micro environment (10). It is generally
accepted that the release of insoluble and fixed forms of P carried out by the action of phosphate-solubilizing
bacteria (PSB) via the secretion of low molecular
weight organic acids mainly gluconic and keto-
gluconic acids and phosphatases (9, 12-15). These
acids are produced in the periplasm of many Gram-
negative bacteria through a direct oxidation pathway of glucose (DOPG, non-phosphorylating oxidation), consequently, the organic acids diffuse freely outside
the cells and may release high amounts of soluble P
from mineral phosphates, by supplying both protons
and metal complex organic acid anions (14, 15).It has already been shown that the use of nitrogen
fixing bacteria with high effective ability in solubilizing organic and inorganic phosphates is one
of the most feasible strategies for the development
of sustainable agriculture. This can supply sufficient N
2 and assist the hydrolysis of a wide range of P
compounds leading to increased crop production
(3, 9, 16). Among the nitrogen-fixing bacteria those belonging to genus Azotobacter, play a significant role (17, 18). Azotobacter is a gamma-proteobacterium
belonging to the family Pseudomonadaceae. It is
an obligate aerobic, free-living Gram-negative,
broadly dispersed in diverse environments, such as
soil, water and sediments (19). In fact, Azotobacter
has beneficial effects on plant yields, due to their ability of fixing nitrogen (6, 18, 20, 21), solubilizing phosphates (4, 16, 22) and to the microbial secretion
of stimulating phytohormones, like gibberellins,
auxins and cytokinins (4, 22). An additional reason which justifies the interest on this microorganism is that the species A. vinelandii and A. chroococcum
produces metabolically dormant cysts. The cyst is
formed under unfavorable environmental conditions
and is the most important basis of application of
Azotobacter strains in various environments (20,
23, 24). The use of native soils Azotobacter is an
important aspect for biofertilizer applications as it is
well adapted to the natural climates of their habitat
and particularly with respect to applications under
harsh environmental conditions.
The aim of this research was to isolate
Azotobacter strains capable of fixing nitrogen and effectively hydrolyzing both organic and inorganic
Pi
compounds. As a new approach, we have set
up a method for screening of Azotobacter with
high phosphatase activity. The effects of several
environmental conditions on propagations and Pi
solubilizing activities of the isolates were assessed too.
MATERIALS AND METHODS
Bacterial isolation and identification. Sixty-four different soil samples from the rhizosphere
of agricultural crop of Iran were transferred to
laboratory (Tehran, Qazvin, Guilan, Mazandaran,
Ardabil and Isfahan). Strategies used for isolation
were: (i) Enrichment of Azotobacter strains, one gram
from each of the soil samples were added into 100 ml
Erlenmeyer flasks containing 20 ml of Azotobacter broth of the following composition; mannitol 20g,
K2HPO
4 0.8 g, KH
2PO
4 0.2 g, MgSO
4·7H
2O 0.5 g,
FeSO4·6H
2O 0.10 g, CaCO
3 20 g, NaMoO
4·2H
2O
0.05 g supplemented with ZnSO4.7H
2O 10 mg,
MnSO4.4H
2O 1.0 mg and cycloheximide (100µg/ml)
per liter (Adjust to pH 7.2). Incubation was at 28°C for 2-5days. (ii) Isolation was carried out by preparing
serial dilutions from enrichment culture followed by
streaking and incubation at 28ºC for 2-5 days. All the
isolates were subcultured on selective nitrogen-free
specific medium Azotobacter Agar plates.Physiological and biochemical characteristics
were performed according to Bergey’s Manual of
Systematic Bacteriology instructions (19), including
colony morphology, the Gram staining, cyst and
PHB granules staining as well as production of
pigment. Molecular identification was performed with PCR using selective nifH-g1 primers fD1
(5’GGTTGTGACCCGAAAGCTGA-3’) and rP1
(5’-GCGTACATGGCCATCATCTC-3’) (5).
Three selected isolates with the highest nitrogen
fixation properties were identified by 16S rDNA amplification and sequencing. Amplifications of the rDNA gene of bacterial strains were carried
NOSRATI ET AL .
IRAN. J. MICROBIOL. Vol. 6, No. (August 2014), 285-295 287 http://ijm.tums.ac.ir
out using universal bacterial primers (fD1) 27F; (AGAGTTTGATCCTGGCTCAG) and (rP1)1492R;
(GGTTACCTTGTTACGACTT)to amplify ˜1.5 kb
product from 16S rDNA (25). Sequence similarities
were analyzed at NCBI GenBank database and
BLAST program at http://www.ncbi.nih.gov/blast.
Phosphate solubilization. To examine Pi
solubilization capabilities, 20 µl of the bacterial suspensions (~104CFU/ml) was spotted on the center of Sperber medium plate containing insoluble P
i.
Sperber medium composed of 10 g glucose, 0.5 g
yeast extract, 0.1 g CaCl2, 0.25 g MgSO
4.7H
2O was
supplemented with 2.5 g Ca3(PO
4)
2 (TCP) and 15
g agar (in solid medium) per liter at pH: 7.2 (26). The inoculated plates were incubated at 28°C. The
diameter of zone of clearance (halo) surrounding the
bacterial colony as well as the diameter of colony
were measured after 2, 4 and 7 days in triplicates. Pi
solubilizing index (PSI) was calculated as the ratio of diameter of halo(mm)/diameter of colony(mm) (27).
All the isolates were also assayed for phosphatase
activity according to the method described by
Malboobi et al. (26). The isolates were cultured on
the Sperber agar medium containing 50 mg/l BCIP
(5-bromo-4-choloro-3-indolyl phosphate; Sigma,
St. Louis, Mo) in the presence of soluble phosphate
(KH2PO
4) or insoluble phosphate (TCP) and
incubated at 28°C. Phosphatase activities in isolates
were compared based on scoring the intensities of
blue stained colonies at 24, 48 and 72 hours.
Nitrogen fixation and Nitrogenase activity. The isolates able to solubilize phosphate on Sperber
agar were used for nitrogen fixation capability. The N-fixing efficiency of these isolates was assessed by the acetylene reduction assay (ARA) as described
by Hardy et al. (28). Briefly, 5ml of the Azotobacter broth in 12 ml vials was inoculated with ~104CFU/ml of each isolate and incubated at 28 ºC for 48-
96 h. Once visible growth was observed, the vials
were plugged with rubber. 10% of the head space
(7 ml) was injected with pure acetylene gas (C2H
2)
after removing an equal amount of air from vials by
means of a disposable plastic syringe (29-31). The gas
samples (0.7 μl) were removed after 24 h incubation, and were assayed for ethylene production with a gas
chromatograph (GOW MAC - GM 816 model). The
chromatograph was fitted with Poropak N column and a H2-FID detector. The rate of nitrogen fixation
was calculated (30, 31) and values were expressed as nanomoles of ethylene produced per hour per vial
(nmoles C2H
4 h−1 vial−1) (31, 32).
Bacterial growth and phosphate solubilization. Two indices, including growth index (GI) which is the logarithm of CFU/ml of culture and Pi solubilizing index (PSI) that represents the amounts of hydrolyzed Pi from TCP, were monitored. Time-
coursed quantitative measurements were carried out
for three isolates with the highest nitrogen fixation (O2, O4 and O6) in 100 ml Erlenmeyer flasks containing 25 ml of Sperber broth medium. 100µl of bacterial suspension (with~104 CFU/ml of each isolate) were inoculated into medium and incubated
at 28°C and 120 rpm. The effect of temperature was
examined by incubation at 15, 28, 37 and 45°C. For high-salt treatments, the extra amounts of salt (1, 2.5, 5 and 7%; w/v) were added to Sperber medium. Initial pH values of medium ranging from 5 to 9
were adjusted by the addition of either KOH or HCl
accordingly. In general, no added NaCl, pH: 7 and temperature of 28°C was used as control conditions.
Sampling was carried out within 96 h. For estimation of the growth rate, 100 μl of medium was removed after 3, 6, 12, 18, 24, 36, 48, 72 and 96 h. Serial dilution were prepared and the growth curve was
drawn, while simultaneous released Pi in culture
supernatant was measured. In brief, 400µl of the medium were removed and centrifuged for 10 min
at 5000 rpm. 200 µl from supernatant were mixed with an equal amount of vanadate-molybdate reagent
and diluted to 2 ml with ddH2O. After 10 min, the
optical densities of the samples were measured at
450 nm against a blank. PSI was estimated using
standard curve. All the experiments were performed in triplicates.
Data analysis and graph drawing were done using
statistics software Graph Pad Prism (v5.0.4) and
Microsoft Excel program.
RESULTS
Bacterial isolation and identification. A total of
43 isolates, named O1 to O43, were selected after
enrichment, isolation and screening from 64 soil
samples. According to biochemical characteristics
and molecular method, three species of Azotobacter,
viz., A. chroococcum, A. virelandii and A. beijerinckii
were identified. Three isolates with the highest
PHOSPHATE SOLUBILIZATION OF NITROGEN STRAINS
288 IRAN. J. MICROBIOL. Vol. 6, No. (August 2014), 285-295 http://ijm.tums.ac.ir
nitrogenase activity, O2, O4 and O6 were chosen for
further characterization. Several biochemical and
biological indicators as well as 16S rDNA sequence
were inspected to figure out the taxonomy of these isolates. The compiled data (not shown) revealed
that O2, O4 and O6 isolates must be strains of
Azotobacter vinelandii. 16S rDNA sequences of
these isolates (GenBank accession Nos. EU935082,
EF620439 and EF634040) were all in agreement with the biochemical tests.
Phosphate solubilization activity. Phosphate
solubilization was measured in all isolates; as shown
in Table 1, twenty isolates were able to produce clear
zone or blue color in minimal medium containing
insoluble Pi or synthetic substrate, respectively, in
the course of plating compared to the other isolates.
In a comparative experiment set up, the largest clear zone was observed for O4 (21.5 mm) followed by
O25 (20.6 mm) within the first 2 days whereas O1, O3 and O32 isolate did not show any clear zone. The PSI
values were increased in the time course of culture of
each isolate. The highest level of PSI was observed
for O4 (3.5±0.1) during 7 days while the lowest rate was for O24 (~1.2) during the same period of time
(Table 1). The strongest blue color was developed by
O4, O17 and O25 within the 24 h and 48h while the others stained weakly (Table 1). Phosphatase activity
was not detected in O9 and O24.
Nitrogen fixation and nitrogenase activity. Nitrogenase activities were determined by checking
the acetylene reduction activity for phosphate
solubilizing isolates. Rates obtained in samples
were in the range of 12.1 to 326.4 nmol C2H
4/h/vial
while cell numbers were adjusted to 107CFU/ml. Five isolates showed the highest N
2-fixing activity.
Three isolates (O2, O4 and O6) were identified as A. vinelandii and the other two strains (O1 and O3)
were A. chroococcum. The acetylene reduction
activity (ARA) result of pure cultures of these
strains is shown in Table 2. A. vinelandi O6 showed
the highest activity (326.4nmol C2H
4/h/vial) followed
by strain O2 with an activity of 265.5nmol C2H
4/h/
vial. Strain O5 had the lowest nitrogenase activity
(12.1nmolC2H
4/h/vial).
Correlation between bacterial growth and phosphate solubilization. Three isolates with
the highest nitrogen fixation (A. vinelandii O2, A.
vinelandii O4 and A. vinelandii O6) were chosen
for further investigations. In a series of preliminary
experiments, the effects of various growth conditions on the survival of these isolates were assessed.
A. vinelandii O2 was capable of growing and Pi
solubilization on solid medium with the added salt of
up to 2.5% between 25 to 45°C and initial pH 6-8. A.
vinelandii O4 was capable of Pi solubilization at 15
to 40°C, pH 5-9 and salt as high as 7% when grown on solid medium. In contrast, A. vinelandii O6 was
able to form colonies at a range of 15-45°C, pH 5-9
and 5% salt. Pi solubilization were observed only at
20-40°C, pH 5-8 and salt concentration of 2.5% for
this strain.
This study was further carried out in liquid
minimal medium for GI and PSI. In all cases, a close
association between GI and PSI was noticeable (Figs. 1-3). Under such conditions, the maximum levels of both GI and PSI for A. vinelandii O2 reached
after 48 h of incubation at 28°C while exponential growth phase of A. vinelandii O6 delayed for 72 h and arrived at stationary phase and maximum PSI (~ 230mg/l) after 72 h. Generally, bacterial growth and consequently PSI of both bacteria were reduced at both
high and low temperatures (Fig. 1).In comparison; the most PSI was obtained for A. vinelandii O4 after
24 h at 37°C (225 mg/l) (Fig. 1). This was gradually decreased when incubation was extended beyond 24 h. Bacterial growth and consequently PSI of this strain
were reduced at 15°C and 45°C (Fig. 1). This was more pronounced for O4 strain such that it never entered
exponential growth phase when incubated at 45°C, although they were alive.
As shown in Fig. 2, increased NaCl concentration in medium was associated with reduced bacterial
growth and PSI values. In other words, the least
amount of both GI and PSI values were detected
at 7% NaCl. A different pattern was observed for O4 strain; P
i solubilization was delayed at low salt
medium and the highest PSI value at 24 h in samples
obtained from medium with 1% salt. PSI values for
O4 were inversely proportional to the GI, particularly
when the added salt was higher than 1%.
Medium inoculation at various initial pH values
resulted in reduced bacterial growth and PSI of O2
and O6 isolates in both alkaline and acidic media.
However, the PSI values for both isolates at acidic
pH were higher than the alkaline pH (Fig. 3).While the growth range of O4 was different at various pH
levels, the initial pH did not affect the PSI values of
NOSRATI ET AL .
IRAN. J. MICROBIOL. Vol. 6, No. (August 2014), 285-295 289 http://ijm.tums.ac.ir
O4 isolate, except for alkaline broth. Maximum PSI was obtained when incubation was extending for more than 72 h. The highest PSI values in acidic and alkaline pH were for O4 strain (Fig. 3).
DISCUSSION
A diverse group of rhizobacteria, collectively
called plant growth promoting rhizobacteria (PGPR),
has been reported to enhance the growth of plants
directly and/or indirectly. As one of the most known
groups of PGPR, nitrogen fixing bacteria have ability of phosphate solublization considered to
main constituents of biofertilizers (3, 9, 16). Of the
most widely used method to study N2 fixation in
asymbiotic systems, the ARA is simpler and faster
than the other methods because it is easily measured
by gas chromatography (20). The results presented in
this paper are in agreement with the previous works.
Rodelas et al.(33) reported the ARA results of pure
Table 1. The solubilization efficiency of Azotobacter strains in plate assay for which Pi solubilizing index (PSI) and presence or absences of phosphatase activity are given
Bacterial strain
Pi solubilizing index (PSI) a Phosphatase activityb, c
2days 4days 7daysWith soluble Pi With insoluble Pi
24 h 48 h 72h 24 h 48 h 72h
O1 0 1.7±0.1 2.1±0.1 + + + + + +
O2 1.4 1.5±0.1 2±0.2 + + + + + +
O3 0 1.7±0.3 2.1±0.1 + + + - + +
O4 2.7±0.2 3.3±0.1 3.5±0.1 ++ ++ ++ ++ ++ ++
O5 1.6±0.1 1.7±0.1 1.8±0.3 + + + + + +
O6 1.6±0.2 1.7±0.1 1.9±0.1 + + + + + +
O8 1.7±0.3 2±0.1 2.4±0.1 + + + - + +
O9 1 1.1±0.1 1.3±0.1 - - - - - -
O11 1.1 1.4±0.2 1.6±0.3 + + + - + +
O14 2.6±0.4 2.8±0.2 3.2±0.1 - - + - - +
O15 1.2±0.1 1.3 1.4±0.1 + + + + + +
O16 1.4 1.7±0.1 1.8±0.2 + + + + + +
O17 2.4 3±0.3 3.4±0.3 ++ ++ ++ + + ++
O21 1.5 1.6±0.2 1.7±0.2 + + + + + +
O24 1 1±0.1 1.2±0.1 - - - - - -
O25 2.2±0.2 2.6±0.2 3 ++ ++ ++ ++ ++ ++
O32 0 1.3±0.2 1.4±0.2 + + + + + +
O33 1.1 1.4±0.2 1.5±0.2 - + + - - -
O34 1.4 1.4 1.6±0.1 + + + + + +
O39 1.7 1.8±0.1 1.8±0.1 + + + + + +
aResults presented are means of three replicates ± standard errorsbSoluble phosphate is KH
2PO
4 and insoluble phosphate is TCP
c + Light blue stain and ++ dark blue stain
Table 2. Acetylene reduction activities in the selected isolates with maximum nitrogenase activity.Values are means of triplicates followed by the standard deviation.
Isolates Nitrogenase activity (nmol C2H
4 h−1 vial−1) Identified strain
O1 87.2 ± 4.6 A.chroococcum
O2 265.5 ± 7.2 A. vinelandii
O3 135.7 ± 3.6 A.chroococcum
O4 158.6 ± 4.7 A. vinelandii
O6 326.4 ± 8.7 A. vinelandii
PHOSPHATE SOLUBILIZATION OF NITROGEN STRAINS
290 IRAN. J. MICROBIOL. Vol. 6, No. (August 2014), 285-295 http://ijm.tums.ac.ir
Fig 1. Bacterial strains O2 (panels A and B), O4 (panels C and D) and O6 (panels E and F) were grown in liquid medium containing tricalcium phosphate. All data points are the means of three replicates. Standard errors are shown by vertical bars.
NOSRATI ET AL .
IRAN. J. MICROBIOL. Vol. 6, No. (August 2014), 285-295 291 http://ijm.tums.ac.ir
Fig 2. Panels are arranged as Fig. 1. All data points are the means of three replicates. Standard errors are shown by vertical bars.
PHOSPHATE SOLUBILIZATION OF NITROGEN STRAINS
292 IRAN. J. MICROBIOL. Vol. 6, No. (August 2014), 285-295 http://ijm.tums.ac.ir
Fig 3. Panels are arranged as Fig. 1. All data points are the means of three replicates. Standard errors are shown by vertical bars.
NOSRATI ET AL .
IRAN. J. MICROBIOL. Vol. 6, No. (August 2014), 285-295 293 http://ijm.tums.ac.ir
cultures of strains were between 9.70 to 257.73nmol C
2H
4 h−1 vial−1. It is worth noting that A. chroococcum
isolates gave high values of ARA. In another study,
Tejera et al. (21) surveyed efficient nitrogen fixation A. chroococcum isolates from soil or rhizosphere
samples with the range of 79.6 to 329.5 nmol C2H
4
h−1 culture.
Although, most studies have focused on biological
nitrogen fixation of Azotobacter but several
researchers paid attention to other characteristic
termed as PGPR. In order to minimize the use of
expensive chemical fertilizers, in this study we investigated the potential of phosphate solubilization
in these strains of Azotobacter. As previously reported
by Kumar et al.(27), Garg et al. (16) and Farajzadeh et
al.(22), the Azotobacter isolates were able to dissolve
inorganic and organic phosphate compounds.
The PSI results obtained in the solid medium was
significantly higher than other observations and corroborated with Farajzadeh et al. (22), indicating
that indigenous strains of Azotobacter isolated
from alkaline soils are effective Pi solubilizers. The
presented results showed that isolated P solubilizing
and N-fixation Azotobacter could serve as efficient biofertilizer candidates for improving the N and P
nutrition of crop plants simultaneously.
Nevertheless, the performance of PGPR as
biofertilizers is severely influenced by both biotic and abiotic environmental conditions of the targeted
regions. This has lead researchers to seek out isolation
of PGPR from native soil and to explore methods for screening PGPR strains and subsequent evaluations
for specific criteria particularly with respect to applications under various environmental conditions.
Tolerances to extreme climates are of special interests for bacteria to be used as biofertilizer in arid and semi-
arid regions. It has been evidenced that the members
of the genus Azotobacter produce copious amount
of capsular slime. This property allows adaptation
of these species to the stressful environments.
Besides, it is suggested that the cyst formation
under unfavorable environmental conditions reduces
oxygen transferring into the cell and consequently increases the nitrogenase activity (23, 34). Therefore,
the isolates were intensively examined for tolerance toward high temperature, high concentration of NaCl
and a wide range of pH (Figs. 1-3). All PSB strains survived under the examined conditions. Decreased GI and PSI at 45°C for the isolate are acceptable as
in natural situation these bacteria survive hot noons
in summer while continue growing on the rest of
the times. All isolate tolerated the added NaCl to
more than 2.5% indicating that these isolates would
functionally be active in most cultivated lands in
which the salinity of soil is usually below 2.5%. As a
key feature of cultivation soils, we measured GI and
PSI of bacteria grown in medium with various initial
pH ranging from 5 to 9. The fact that the highest GI
and PSI values for isolates were obtained at high pH
suggests that these bacteria favor alkaline condition.
These results indicate that the isolated Azotobacter
could be used as biofertilizer in agricultural soils
in Iran which are predominately calcareous and are
characterized by a high pH and low amounts of plant
available phosphorus (35).
Islam et al. (36) investigated the effects of pH,
temperature and salt on growth of Azotobacter
spp. isolated from crop rhizosphere. Even though
no data was presented for PSI, they however
showed tolerance for a number of their isolated
strains toward different environmental factors
such as NaCl (0-0.8%) and temperature (20– 40°C)
while no isolate survived at 50ºC. Rajeswari et al.
(37) isolated Azotobacter species which showed
tolerance toward the extra amounts of salt (3.5%) in an optimized medium. Tejera et al. (21) assessed
growth rates at different initial pH values (4- 9) and
showed that a lower number of isolates grew on N-
free media at pH value as high as 8.7. Ravikumar et
al. (31) investigated the effects of salinity on nitrogen
fixation and also indole acetic acid (IAA) production They found a maximum level of IAA production and nitrogenease activity at the salinity of 20 g/l and 30
g/l NaCl, respectively.
A noticeable reduction in PSI during stationary
phase of growth found in all cases supports the
dependence of PSI on bacterial metabolism. It was
also shown that the phosphatase activity of bacterial
strain could synergistically enhance the release of
Pi in the acidified medium (Fig. 3). The advantage of bacteria capable of phosphate solubilitizing
with simultaneous secretion of organic acids and
phosphatase activity on production and yield were
shown in both green house and field trials (38). In conclusion, we have isolated several Azotobacter
strains, three of which were more promising for being
used as N and P biofertilizer. These bacteria are well
adapted to various environmental conditions.
PHOSPHATE SOLUBILIZATION OF NITROGEN STRAINS
294 IRAN. J. MICROBIOL. Vol. 6, No. (August 2014), 285-295 http://ijm.tums.ac.ir
REFERENCES
1. Hofman G, van Cleemput O ( 2004). Soil and
Plant Nitrogen. 1 ed. IFA. Paris. 2. Rees DC, Akif Tezcan F, Haynes CA, Walton
MY, Andrade S, Einsle O et al. Structural basis
of biological nitrogen fixation. Phil Trans R Soc
2011; 363:971.3. Richardson AE, Barea JM, McNeill AM,
Prigent-Combaret C. Acquisition of phosphorus
and nitrogen in the rhizosphere and plant growth
promotion by microorganisms. Plant Soil 2009;
321:305-339.
4. Hayat R, Ali S, Amara U, Khalid R, Ahmed I.
Soil beneficial bacteria and their role in plant growth promotion: a review. Ann Microbiol 2010;
60:579-598.5. Burgmann H, Widmer F, Von Sigler W, Zeyer
J. New molecular screening tools for analysis
of free-living diazotrophs in soil. Appl Environ
Microbiol 2004; 70:240-247.6. Dixon R, Kahn D. Genetic regulation of biological
nitrogen fixation. Nat Rev Microbiol 2004; 2:621-
631.
7. Halbleib CM, Ludden PW. Regulation of biological nitrogen fixation. The Journal of Nutrition 2000; 130:1081.
8. Raymond J, Siefert JL, Staples CR, Blankenship
RE. The natural history of nitrogen fixation. Mol
Biol Evol 2004; 21:541.
9. Rodriguez H, Fraga R. Phosphate solubilizing bacteria and their role in plant growth promotion.
Biotechnol Adv 1999; 17:319-339.10. Deubel A, Merbach W (2005). Influence of
Microorganisms on Phosphorus Bioavailability in
Soils Microorganisms in Soils: Roles in Genesis
and Functions. A Varma, F Buscot. Springer Berlin Heidelberg, 3 ed, pp. 177-191.
11. Mehta S, Nautiyal CS. An efficient method for qualitative screening of phosphate-solubilizing
bacteria. Curr Microbiol 2001; 43:51-56.
12. Chung H, Park M, Madhaiyan M, Seshadri S,
Song J, Cho H et al. Isolation and characterization
of phosphate solubilizing bacteria from the
rhizosphere of crop plants of Korea. Soil Biol
Biochem 2005; 37:1970-1974.13. Khan MS, Zaidi A, Wani PA (2009). Role of
Phosphate Solubilizing Microorganisms in
Sustainable Agriculture Sustainable Agriculture.
E Lichtfouse, M Navarrete, P Debaeke, S
Vinronique, C Alberola. Springer Netherlands,
pp. 551-570.14. Perez E, Sulbaran M, Ball MM, Yarzabal
LA. Isolation and characterization of mineral
phosphate-solubilizing bacteria naturally
colonizing a limonitic crust in the south-eastern
Venezuelan region. Soil Biol Biochem 2007; 39:2905-2914.
15. Sashidhar B, Podile AR. Mineral phosphate
solubilization by rhizosphere bacteria and scope
for manipulation of the direct oxidation pathway involving glucose dehydrogenase. J Appl
Microbiol 2010; 109:1-12.
16. Garg SK, Bhatnagar A, Kalla A, Narula N. In
vitro nitrogen fixation, phosphate solubilization, survival and nutrient release by Azotobacter
strains in an aquatic system. Bioresource
Technology 2001; 80:101-109.
17. Fischer S, Fischer S, Magris S, Mori G. Isolation and characterization of bacteria from
the rhizosphere of wheat. World J Microbiol
Biotechnol 2007; 23:895-903.18. Aquilanti L, Favilli F, Clementi F. A comparison of
different strategies for isolation and preliminary
identification of Azotobacter from soil samples.
Soil Biol Biochem 2004; 36:1475-1483.19. Staley J, Brenner D, Krieg N (2005).
The Proteobacteria, Part B The
Gammaproteobacteria. In: Bergey’s manual
of systematic bacteriology. Ed, GM Garrity.
Springer, 2nd ed, Michigan, pp. 384-402.
20. Sar bay GF. Growth and nitrogen fixation dynamic of Azotobacter chroococcum in nitrogen-free
and omw containing medium [ Department of
Food Engineering,The Middle East Technical University; 2003.
21. Tejera N, Lluch C, Martinez-Toledo MV,
Gonzalez-Lopez J. Isolation and characterization
of Azotobacter and Azospirillum strains from the
sugarcane rhizosphere. Plant Soil 2005; 270:223-232.
22. Farajzadeh D, Yakhchali B, Aliasgharzad N, Sokhandan-Bashir N, Farajzadeh M. plant growth promoting characterization of indigenous
Azotobacteria Isolated from Soils in Iran. Curr
Microbiol 2012; 64:397-403.23. Sabra W, Zeng AP, Lunsdorf H, Deckwer WD.
Effect of oxygen on formation and structure of Azotobacter vinelandii alginate and its role in
protecting nitrogenase. Appl Environ Microbiol
2000; 66:4037.24. Daiz-Barrera A, Soto E. Biotechnological uses of
Azotobacter vinelandii: Current state, limits and
prospects. Afr J Biotechnol 2010; 9:5240-5250.
25. Reid NM, Bowers TH, Lloyd-Jones G. Bacterial
community composition of a wastewater
treatment system reliant on N2 fixation. Appl
Microbiol Biotechnol 2008; 79:285-292.
NOSRATI ET AL .
IRAN. J. MICROBIOL. Vol. 6, No. (August 2014), 285-295 295 http://ijm.tums.ac.ir
26. Malboobi MA, Owlia P, Behbahani M, Sarokhani
E, Moradi S, Yakhchali B et al. Solubilization of
organic and inorganic phosphates by three highly
efficient soil bacterial isolates. World Journal of
Microbiology and Biotechnology 2009; 25:1471-1477.
27. Kumar V, Narula N. Solubilization of inorganic phosphates and growth emergence of wheat as
affected by Azotobacter chroococcum mutants.
Biol Fertil Soils 1999; 28:301-305.
28. Hardy RWF, Holsten RD, Jackson EK, Burns RC. The acetylene-ethylene assay for N2 fixation: laboratory and field evaluation. Plant Physiology
1968; 43:1185.
29. Muthukumarasamy R, Revathi G,
Lakshminarasimhan C. Influence of N fertilisation on the isolation of Acetobacter
diazotrophicus and Herbaspirillum spp. from
Indian sugarcane varieties. Biol Fertil Soils 1999;
29:157-164.30. Natheer SE, Muthukkaruppan S. Assessing
the in vitro zinc solubilization potential and
improving sugarcane growth by inoculating
Gluconacetobacter diazotrophicus. Ann
Microbiol 2012; 62:435-441.
31. Ravikumar S, Kathiresan K, Ignatiammal S,
Babu Selvam M, Shanthy S. Nitrogen-fixing Azotobacters from mangrove habitat and their
utility as marine biofertilizers. J Exp Mar Biol
Ecol 2004; 312:5-17.
32. Methods for studing biological soil crusts.
Methodology Paper of the 4th International
Conference on IL TER East Asian and Pacific Region; 2001.
33. Rodelas B, Gonzalez-Lopez J, Pozo C, Salmeron
V, Martinez-Toledo MV. Response of faba bean
(Vicia faba L.) to combined inoculation with
Azotobacter and Rhizobium leguminosarum bv.
viceae. Appl Soil Ecol 1999; 12:51-59.
34. Nosrati R, Owlia P, Saderi H, Olamaee M, Rasooli
I, Akhavian Tehrani A. Correlation between
nitrogen fixation rate and alginate productivity of an indigenous Azotobacter vinelandii from Iran .
Iran J Microbiol 2012; 4:153-159.
35. Alikhani HA, Saleh-Rastin N, Antoun H.
Phosphate solubilization activity of rhizobia
native to Iranian soils. Plant Soil 2006; 287:35-41.
36. Islam M, Sharif D, Hossain M. A Comparative
study of Azotobacter spp. from different soil
samples. J Soil Nat 2008; 2:16-19.
37. Rajeswari K, Kasthuri M. Molecular characterization of Azotobacter spp. nifH gene
isolated from marine source. Afr J Biotechnol
2009; 8:6850-6855.
38. Malboobi MA, Behbahani M, Madani H, Owlia
P, Deljou A, Yakhchali B et al. Performance
evaluation of potent phosphate solubilizing
bacteria in potato rhizosphere. World J Microbiol
Biotechnol 2009; 25:1479-1484.
PHOSPHATE SOLUBILIZATION OF NITROGEN STRAINS